1) Field of the Invention
This invention relates generally to fabrication of a semiconductor device and more particularly to a method for forming binary intensity masks (BIM) which provides reduced exposure time and improved uniformity of critical dimensions (CD).
2) Description of the Prior Art
Binary intensity masks (binary masks, chrome-on-glass, BIM) are widely used in semiconductor fabrication. Typically, a layer of opaque material, such as chrome, is deposited on a transparent substrate material, such as quartz. The opaque layer is then patterned using an E-beam process. Next, a semiconductor structure or wafer is coated with a layer of photosensitive material and light energy, is transmitted through the binary mask onto the semiconductor structure, exposing the photosensitive layer in areas underlying where the opaque material has been removed. The photosensitive layer is then exposed to a chemical developing solution which, depending on the type of photosensitive material, removes either the exposed areas of the photosensitive layer or the unexposed areas of the photosensitive layer. In this way layers of a semiconductor device, particularly polysilicon layers, can be patterned.
However, the typical BIM process suffers from several disadvantages. As device geometries continue to shrink and device densities increase, critical dimension uniformity must increase to prevent defects. Also, exposure time for patterning increases.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering the following patents.
U.S. Pat. No. 5,888,678 (Tzu et al.) Teaches a mask and a method for forming an alternating phase shift mask (APSM)and a binary intensity mask (BIM) on the same mask.
U.S. Pat. No. 5,288,569 (Lin)teaches feature biasing and absorptive phase shift techniques.
U.S. Pat. No. 5,807,649 (Liebmann et al.) Discloses a phase trim mask with dimensions increased to remove defects.
U.S. Pat. No. 5,801,954 (le et al.)describes a method for designing a BIM.
U.S. Pat. No. 5,923,566 (Galan et al.) Teaches a phase shift design verification routine.
It is an object of the present invention to provide a method for forming a binary intensity mask with reduced exposure time and greater critical dimension uniformity.
It is another object of the present invention to provide a method for forming a binary intensity mask with reduced probability of mask defects.
It is yet another object of the present invention to provide a method for forming a binary intensity mask with reduced E-beam proximity effect.
To accomplish the above objectives, the present invention provides a method for forming a binary intensity mask using a two-step writing process. The first writing step has a narrow writing area, preferably about 1 micron, and outlines the desired pattern (which is not exposed). The second writing area partially overlaps the first writing area, preferably by less than half of the E-beam diameter used for writing, and extends opposite the desired pattern. The second writing does not overlap the desired pattern. The chromium layer of the BIM is dry etched after the first writing, providing good edge definition and dimensional stability. The chromium layer of the BIM is wet etched following the second writing reducing mask defects.
The present invention provides considerable improvement over the prior art. The present invention provides a method for forming a BIM with a reduced exposure time, reducing fabrication costs and cycle time. The present invention provides greater critical dimension uniformity, increasing yield. Also, because the majority of the chromium layer, which is removed, is wet etched, mask defects are reduced.